Optical resonators for lasers generally consist of two mirrors, separated by a distance and facing each other. A gain medium between the mirrors amplifies light confined by the mirrors, and resonant frequencies are amplified during feedback to produce laser light. An important resonator design is the Fabry-Perot or parallel-plane cavity resonator, which consists of two planar mirrors. These resonators are used in lasers for many applications for favorable optical properties, including large free spectral range and high finesse. However, the planar mirror Fabry-Perot resonators suffer from the drawback of being “critically stable”. Slight misalignments of the two planar mirrors (i.e., are not absolutely parallel) cause the cavity to become unstable. This makes lasing difficult, if at all possible. Fabry-Perot resonators, therefore, have little to no manufacturing tolerance. Furthermore, heating of a laser gain medium will adversely impact beam quality. Thermal gradients, arising from heat dissipation in high-power lasers, cause refractive index variations in the gain medium which result in the degradation of beam quality when a single beam is produced.